The invention relates to a covering cap for a gas bag module of a vehicle occupant restraint device.
Such covering caps are intended to protect the gas bag lying underneath from being damaged before it is used, but at the same time are intended to match the appearance of the interior of the vehicle in an optimum manner. Therefore the covering cap is often matched in material and structure for example to the design of the instrument panel. On activation of a gas bag of a vehicle occupant restraint system, the covering cap, which is integrated for example in the steering wheel or in the instrument panel, must be opened in order to free the gas bag. Often for this purpose predetermined breaking points are prefabricated in the covering caps, at which the covering cap can open.
In the case of covering caps consisting of plastic, there is a problem in that the forces necessary for opening the covering cap vary with the temperature, because the material characteristics of the plastics which are used are highly dependent on temperature. Thus, for example, the brittleness of the plastic increases greatly as the temperature falls, whereas the strength of the plastic decreases with a rising temperature. This leads to the covering cap, which is designed for an optimum opening in summertime temperatures not being able to be reliably opened at low temperatures in winter. In addition, in the case of low temperatures, already small external forces can lead to a damage to the covering cap.
The invention provides a covering cap for a vehicle occupant restraint system which fulfills its function reliably over a wide temperature range. This is achieved in a covering cap which comprises at least one plastic layer and is provided at least in places with a heating means, in particular an electric heating arrangement. Since therefore at lower temperatures the temperature of the covering cap can be increased with respect to the ambient temperature, the material characteristics of the plastic of the covering cap are kept constant over a wide range of the ambient temperature, and the force necessary for opening the covering cap remains approximately constant. As the covering cap is therefore always kept at a defined temperature, the notch sensitivity, which increases with a falling temperature, also does not increase with low ambient temperatures.
In an advantageous further development of the invention, the covering cap has a weakened zone and the heating means is arranged in the region of the weakened zone. A weakened zone, for example a tear line or a zone having reduced thickness in the covering gap, is arranged at the site at which the covering cap is to open in order to free the gas bag. This is the actually critical zone of the covering cap, because it must be ensured that at this site the covering cap opens on application of a defined force. Hence it is therefore sufficient to arrange the heating means only in the region of this weakened zone, e.g. directly at the weakened zone or in sections of the covering cap lying immediately adjacent thereto. In this way, the production becomes less costly and less intensive with respect to material.
Advantageously, the heating means is a heating wire. As the weakened zone generally does not have a large width, the use of a favorably priced heating wire is sufficient to heat this area.
Preferably the heating means consists of an electrically conductive plastic. An electrically conductive plastic can be easily included in the desired arrangement and layer thickness during the manufacturing process of the covering cap consisting of plastic. Also, the danger does not exist either that the layer presses through the material and impairs the visual appearance.
Advantageously, a layer of electrically conductive plastic is embedded during the manufacturing process of the covering cap. In this way, the heating means can be produced in one manufacturing step with the covering cap.
The electrical connecting elements for connection of the heating means to a current source are preferably embedded in a plastic layer of the covering by casting, in order to keep the production costs low.
In another advantageous embodiment of the invention, the heating means is a heating mat. This is particularly advantageous owing to the flat extent of a heating mat when the entire covering cap is to be heated.
For simple and close fastening of the heating mat to the covering, the heating mat is preferably connected to a plastic layer of the covering by ultrasonic welding.
In an advantageous embodiment, a leather coating is provided, and the heating means is arranged between the plastic layer of the covering and the leather coating. In this way, a leather-covered covering cap can be heated in a simple manner.